If there are 30 chromosomes in `G_(1)` -phase, then what will be number of bivalents in zygotene stage ?

Options

A. 30
B. 15
C. 45
D. 60

Answer

To solve the question regarding the number of bivalents in the zygotene stage when there are 30 chromosomes in the G1 phase, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Chromosome Number in G1 Phase**:
- In the G1 phase, the cell has 30 chromosomes. This is the diploid number (2n) for the organism, meaning there are 30 individual chromosomes present.

2. **DNA Replication in S Phase**:
- After the G1 phase, the cell enters the S phase where DNA replication occurs. However, the number of chromosomes remains the same (30), but each chromosome now consists of two sister chromatids. Thus, while the DNA content doubles, the chromosome count remains 30.

3. **Transition to Zygotene Stage**:
- The zygotene stage is part of prophase I of meiosis, where homologous chromosomes begin to pair up. Each pair of homologous chromosomes forms a structure known as a bivalent or tetrad.

4. **Formation of Bivalents**:
- Since there are 30 chromosomes, they can be paired into homologous pairs. Each pair consists of 2 chromosomes. Therefore, to find the number of bivalents, we divide the total number of chromosomes by 2:
\[ \text{Number of bivalents} = \frac{\text{Total Chromosomes}}{2} = \frac{30}{2} = 15 \]

5. **Conclusion**:
- Therefore, in the zygotene stage, the number of bivalents formed will be 15.

### Final Answer:
The number of bivalents in the zygotene stage is **15**.

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